Whatever you make of them, the three glass-and-steel spheres Amazon.com has proposed as the “heart” of its high-rise complex in Seattle’s Denny Triangle drew instant reaction Tuesday from online commenters and at a public hearing before a city design review board.

The structure would feature five floors of flexible work space totaling 65,000 square feet and would be “capable of accommodating mature trees,” according to plans filed with the city. The spheres would range in height from 80 feet to 95 feet.

“There’s something optimistic, forward-looking, experimental, exciting, whimsical about this building,” said Gabe Grant, a review board member who is vice president of Seattle-based HAL Real Estate Investments.

Mathew Albores, an architect on the board, wasn’t impressed. For pedestrians on the sidewalk, the spheres offer a similar urban experience to the EMP Museum and the Seattle Public Library, with scarce retail and no overhead rain protection, he said. And why, he asked, have more trees inside the spheres where the public won’t be allowed than outside in the adjacent public space?

Amazon wants to build a total 3.3 million square feet of office space on the three blocks it bought in December 2012 for $207.5 million. If completed, the development would be downtown’s largest ever.

Amazon’s campus, which the company has nicknamed “Rufus 2.0” after a former employee’s dog, would feature six buildings on each block: three 37- or 38-story high-rise towers and three smaller buildings — one of which would be the spheres.

The spheres would feature plants from high-elevation climates around the world that can thrive in an office kept between 68 and 72 degrees Fahrenheit. At night, the temperature in the building would be lowered and the humidity raised so the plants are comfortable, said John Savo, a principal at NBBJ, Amazon’s architect on the project.

The glazed glass-and-steel structure would be on the block between Sixth and Seventh avenues and between Blanchard and Lenora streets. Previously, Amazon had proposed a six-story rectangular office building there.

“We all know what a typical office space is like,” said Savo. “Wouldn’t it be cool to go into a building that’s more like a park and be comfortable?”

The architects sought to create an experimental work space that’s an amenity for all Amazon employees, he explained to the review board: It’s a place where individuals, teams or departments can go to brainstorm. Think treehouse offices.

But it’s not envisioned as a place for the public to roam, except to look in from the outside, he said. Retail spaces on Sixth and Seventh avenues would be open to the public, however.

There will also be public open spaces, such as a play field and dog park, in between the spheres and the office tower next door on what Amazon calls “Block 19.”

Amazon.com expects to finish that block by late 2016, he said.

The largest dome would be 130 feet in diameter, the smallest about 80 feet in diameter.

NBBJ’s team showed photos of conservatories and gardens around the world during Tuesday’s hearing, including:

• Ottawa’s Convention Center, an oval glass building that opened in April 2011.

• Milwaukee’s Mitchell Park Conservatory, three aluminum-and-glass domes covering one acre that opened in the 1960s.

• A waterfront biosphere named “La Bolla” in Genoa, Italy.

• The Royal Greenhouses of Laeken in Brussels, Belgium, a 6-acre attraction.

• Singapore’s Gardens by the Bay, a 250-acre park on reclaimed land that opened last summer.

• Nabana no Sato, a botanical garden on the island of Nagashima in Japan.

“We’re not building on the scale of Gardens by the Bay in Singapore, but we’re inspired by this imagery,” Savo said.

Design review board member Gundula Proksch, an assistant professor of architecture at the University of Washington, said greenhouses can be “very energy intensive” year-round and seemed unsatisfied by NBBJ’s assertions that the building would seek LEED Gold certification, a designation by the U.S. Green Building Council.

Biospheres aren’t new, Proksch said.

“What is it offering other than making an experimental work environment?” she asked.

It’s better than the boxy building Amazon had previously proposed, Savo said.

The striking glass biospheres would be a refreshing departure from the “very vanilla” buildings typical of Seattle’s urban landscape, said local developer Kevin Daniels.

“Most of our buildings have been designed by bean-counters,” he said. Around the world, but especially in China and the Middle East, “there are such exceptional architectural designs that are improving the cities they reside in, and we have very little of that here.”

Daniels said Amazon “has taken the time to really look at how they can improve the neighborhood and city by using architectural design.”

The proposed spheres, intended to give employees a place to “work and socialize in a more natural, parklike setting” but undoubtedly also to stand out amid downtown’s towers, do seem contrary to Amazon’s traditional low profile.

Its South Lake Union headquarters has not a single “Amazon” sign or logo on the outside.

There was no Amazon logo apparent in the renderings of the spheres either.

But perhaps the very idea of having a selection of the world’s plants gathered under three large temperature-controlled domes is enough to signal this is no wallflower.

“We’re striving to create an environment in which to work and think a little differently,” Savo said.

Seattle Times business reporter Amy Martinez contributed to this report.

This story, published May 21, 2013, was updated May 22 to clarify comments by architect Mathew Albores. He said the pedestrian experience, not the style, of the Amazon spheres would be similar to that of the EMP Museum and the Seattle Public Library.